feat(checkout): add ability to dry run checkout

lib/src/git_types.dart

@@ -550,6 +550,10 @@ class GitCheckout {
   /// Normally checkout writes the index upon completion; this prevents that.
   static const dontWriteIndex = GitCheckout._(8388608, 'dontWriteIndex');
 
+  /// Show what would be done by a checkout. Stop after sending
+  /// notifications; don't update the working directory or index.
+  static const dryRun = GitCheckout._(16777216, 'dryRun');
+
   static const List<GitCheckout> values = [
     none,
     safe,
@@ -571,6 +575,7 @@ class GitCheckout {
     conflictStyleDiff3,
     dontRemoveExisting,
     dontWriteIndex,
+    dryRun
   ];
 
   int get value => _value;

test/checkout_test.dart

@@ -1,4 +1,5 @@
 import 'dart:io';
+import 'package:libgit2dart/src/git_types.dart';
 import 'package:test/test.dart';
 import 'package:libgit2dart/libgit2dart.dart';
 import 'helpers/util.dart';
@@ -93,5 +94,20 @@ void main() {
         },
       );
     });
+
+    test('successfully performs dry run checkout', () {
+      final index = repo.index;
+      expect(index.length, 4);
+      expect(File('${repo.workdir}/another_feature_file').existsSync(), false);
+
+      repo.checkout(
+        refName: 'refs/heads/feature',
+        strategy: {GitCheckout.dryRun},
+      );
+      expect(index.length, 4);
+      expect(File('${repo.workdir}/another_feature_file').existsSync(), false);
+
+      index.free();
+    });
   });
 }